U.S. patent application number 11/446333 was filed with the patent office on 2006-10-05 for perovskite catalyst system for lean burn engines.
This patent application is currently assigned to Ford Global Technologies, Inc.. Invention is credited to Haren S. Gandhi, Ronald Gene Hurley, Jun (John) Li.
Application Number | 20060223694 11/446333 |
Document ID | / |
Family ID | 32106054 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060223694 |
Kind Code |
A1 |
Gandhi; Haren S. ; et
al. |
October 5, 2006 |
Perovskite catalyst system for lean burn engines
Abstract
A catalyst system for use with an internal combustion engine to
provide emissions reductions under lean and stoichiometric
operating conditions. The catalyst system comprises a first
catalyst comprised of a newly developed Perovskite-based
formulation having an ABO.sub.3 crystal structure designed to bring
the precious metal and NOx trapping elements close together. The
first catalyst acts primarily to maximize the reduction of
emissions under lean operating conditions. The catalyst system also
comprises a second catalyst comprised of precious metals which acts
primarily to maximize the reduction of emissions under
stoichiometric conditions.
Inventors: |
Gandhi; Haren S.; (West
Bloomfield, MI) ; Li; Jun (John); (Canton, MI)
; Hurley; Ronald Gene; (Plymouth, MI) |
Correspondence
Address: |
BROOKS KUSHMAN P.C./FGTL
1000 TOWN CENTER
22ND FLOOR
SOUTHFIELD
MI
48075-1238
US
|
Assignee: |
Ford Global Technologies,
Inc.
Dearborn
MI
|
Family ID: |
32106054 |
Appl. No.: |
11/446333 |
Filed: |
June 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10065498 |
Oct 24, 2002 |
7071141 |
|
|
11446333 |
Jun 2, 2006 |
|
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Current U.S.
Class: |
502/60 |
Current CPC
Class: |
B01D 2255/1025 20130101;
Y02T 10/12 20130101; C01G 51/68 20130101; B01J 23/63 20130101; B01D
2255/20738 20130101; C01P 2006/12 20130101; C01P 2002/77 20130101;
C01P 2002/34 20130101; B01D 53/9418 20130101; C01G 51/006 20130101;
Y02T 10/24 20130101; B01J 23/002 20130101; B01D 2255/2047 20130101;
Y02T 10/22 20130101; C01G 55/002 20130101; B01D 2255/402 20130101;
B01D 2255/20761 20130101; B01J 2523/00 20130101; B01D 2255/20746
20130101; B01D 2255/2022 20130101; B01D 2255/1021 20130101; B01D
2255/2042 20130101; C01G 49/009 20130101; B01J 23/8946 20130101;
C01P 2002/52 20130101; B01J 37/03 20130101; B01D 53/945 20130101;
B01J 2523/00 20130101; B01J 2523/25 20130101; B01J 2523/3706
20130101; B01J 2523/822 20130101; B01J 2523/845 20130101; B01J
2523/00 20130101; B01J 2523/25 20130101; B01J 2523/3706 20130101;
B01J 2523/828 20130101; B01J 2523/842 20130101; B01J 2523/845
20130101; B01J 2523/00 20130101; B01J 2523/25 20130101; B01J
2523/3706 20130101; B01J 2523/828 20130101; B01J 2523/845
20130101 |
Class at
Publication: |
502/060 |
International
Class: |
B01J 29/04 20060101
B01J029/04 |
Claims
1. A method of reducing emissions from an exhaust gas stream
comprising: providing a first catalyst for optimizing the storage
of NO.sub.x emissions under lean air/fuel ratios, comprising a
Perovskite-type ABO.sub.3 crystal structure wherein the A cation
sites are occupied by lanthanide ions and the B cation sites are
occupied by cobalt ions, wherein from about 1 to up to 70% of the
lanthanide A cation sites are substituted with a NO.sub.x trapping
metal selected from the group consisting of barium, magnesium and
potassium, wherein from about 1 to up to 60% of the cobalt B cation
sites are substituted with a metal selected from the group
consisting of platinum, rhodium, iron, copper and manganese; and
providing a second catalyst for optimizing the reduction of
hydrocarbon, NO.sub.x and CO emissions under stoichiometric
air/fuel ratios comprising a catalyst mixture PM-Rh wherein PM is a
catalyst material selected from the group consisting of platinum,
palladium and combinations thereof wherein the first and second
catalysts are closely coupled, the first catalyst being placed in a
forward position and the second catalyst being placed in a
downstream position in the exhaust stream.
2. The method of claim 1 wherein the first catalyst is prepared by
sol-gel.
3. The method of claim 1 wherein the first catalyst is prepared by
co-precipitation.
4. The method of claim 1, wherein the ratio of PM:Rh in the
catalyst mixture PM-Rh is 9:1.
5. The method of claim 1 wherein the ratio of PM:Rh in the catalyst
mixture PM-Rh is 7:1.
6. The method of claim 1 wherein the PM has a total loading of
20-60 g/ft.sup.3.
7. The method of claim 1 wherein the PM has a total loading of
40-60 g/ft.sup.3.
8. The method of claim 1, wherein the first catalyst has the
formula La.sub.0.5Ba.sub.0.5Co.sub.0.9Rh.sub.0.1O.sub.3.
9. The method of claim 1 wherein the first catalyst has the formula
La.sub.0.5Ba.sub.0.5Co.sub.0.6Fe.sub.0.3Pt.sub.0.1O.sub.3.
10. The method of claim 1 wherein the first catalyst has the
formula La.sub.0.5Ba.sub.0.5Co.sub.0.9Pt.sub.0.1O.sub.3.
11. The method of claim 1 wherein the catalyst mixture PM-Rh is
coated on an alumina substrate.
12. The catalyst system of claim 11, wherein the alumina substrate
in the second catalyst is stabilized by 2-20% (wt) BaO.
13. The catalyst system of claim 11, wherein the PM is loaded on
the alumina substrate by wet impregnation.
14. The method of claim 1, wherein the platinum and rhodium in the
second catalyst are placed on Ce and Zr particles of 2-20 wt %.
15. The method of claim 1 wherein an exhaust gas sensor is placed
between the first and second catalysts.
16. A method of reducing emissions from an exhaust gas stream
comprising: providing a first catalyst for optimizing the storage
of NO.sub.x emissions under lean air/fuel ratios, comprising a
Perovskite-type ABO3 crystal structure wherein the A cation sites
are occupied by lanthanide ions and the B cation sites are occupied
by cobalt ions, wherein from about 1 to up to 70% of the lanthanide
A cation sites are substituted with a NO.sub.x trapping metal
selected from the group consisting of barium, magnesium, and
potassium, wherein from about 1 to up to 60% of the cobalt B cation
sites are substituted with a metal selected from the group
consisting of platinum, rhodium, iron, copper and manganese;
providing a second catalyst for optimizing the reduction of
hydrocarbon, NO.sub.x and CO emissions under stoichiometric
air/fuel ratios, comprising a catalyst mixture PM-Rh where PM is a
catalyst material selected from the group consisting of platinum,
palladium and combinations thereof; and placing an exhaust gas
sensor between the first and second catalysts.
17. The method of claim 16 wherein the first catalyst has a formula
selected from the group consisting of
La.sub.0.5Ba.sub.0.5CO.sub.0.9Rh.sub.0.1O.sub.3,
La.sub.0.5Ba.sub.0.5Co.sub.0.6Fe.sub.0.3Pt.sub.0.1O.sub.3, and
La.sub.0.5Ba.sub.0.5CO.sub.0.9Pt.sub.0.1O.sub.3.
18. The method of claim 16 wherein the ratio of PM:Rh in the
catalyst mixture PM-Rh is 9:1.
19. The method of claim 1 wherein the PM has a total loading of
20-60 g/ft.sup.3.
20. A method of reducing emissions from an exhaust gas stream
comprising: providing a first catalyst for optimizing the storage
of NO.sub.x emissions under lean air/fuel ratios, comprising a
Perovskite-type ABO.sub.3 crystal structure wherein the A cation
sites are occupied by lanthanide ions and the B cation sites are
occupied by cobalt ions, wherein from about 1 to up to 70% of the
lanthanide A cation sites are substituted with a NO.sub.x trapping
metal selected from the group consisting of barium, magnesium and
potassium, wherein from about 1 to up to 60% of the cobalt B cation
sites are substituted with a metal selected from the group
consisting of platinum, rhodium, iron, copper and manganese;
providing a second catalyst for optimizing the reduction of
hydrocarbon, NO.sub.x and CO emissions under stoichiometric
air/fuel ratios comprising a catalyst mixture PM-Rh wherein PM is a
catalyst material selected from the group consisting of platinum,
palladium and combinations thereof wherein the first and second
catalysts are closely coupled, the first catalyst being placed in a
forward position and the second catalyst being placed in a
downstream position in the exhaust stream; and placing an exhaust
gas sensor between the first and second catalysts.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/065,498 filed Feb. 7, 2006.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to a catalyst system for
use with internal combustion engines to oxidize hydrocarbons,
carbon monoxide and reduce nitrogen oxides in an exhaust gas when
the engine is operated at both lean and stoichiometric air/fuel
ratios. More particularly, the catalyst system of this invention
includes two catalysts. The first catalyst is designed specifically
to optimize the reduction of noxious emissions under lean
conditions. This first catalyst includes a new Perovskite-based
formulation designed to achieve close proximity between precious
metal and NOx binding elements.
[0004] The second catalyst is designed specifically to maximize the
reduction of HC, CO and NOx under stoichiometric operations and to
treat any breakthrough NOx emissions from the first catalyst. This
second catalyst contains precious metals and may optionally include
BaO.
[0005] 2. Background Art
[0006] Catalysts have long been used in the exhaust systems of
automotive vehicles to convert carbon monoxide, hydrocarbons, and
nitrogen oxides (NOx) produced during engine operation into
nonpolluting gases including carbon dioxide, water and nitrogen.
When a gasoline-powered engine is operated at a stoichiometric or
slightly rich air/fuel ratio, i.e., between about 14.6 and 14.4,
catalysts containing precious metals like platinum, palladium and
rhodium are able to efficiently convert all three gases
simultaneously. Typically, such catalysts use a relatively high
loading of precious metal to achieve the high conversion efficiency
required to meet the stringent emission standards of many
countries. Because of the high cost of the precious metals, these
catalysts are expensive to manufacture.
[0007] To improve vehicle fuel efficiency and lower CO.sub.2
emissions, it is preferable to operate an engine under lean
conditions. Lean conditions are air/fuel mixtures greater than the
stoichiometric mixture (an air/fuel mixture of 14.6), typically
air/fuel mixtures greater than 15. While lean operation improves
fuel economy, operating under lean conditions increases the
difficulty in treating some polluting gases, especially NOx.
[0008] For some catalysts, if the air/fuel ratio is lean even by a
small amount, NOx conversion is significantly reduced. One way to
provide air/fuel control is through the use of a HEGO (Heated
Exhaust Gas Oxygen) sensor to provide feedback to the control
systems. HEGO sensors, however, over time develop a lean bias as a
result of poisoning. Accordingly, even with a HEGO sensor it is
important to have a catalyst that can maximize the reduction of NOx
emission under lean conditions.
[0009] To maximize NOx reduction under lean operating conditions, a
lean NOx trap is often used. The inclusion of a NOx trap enhances
the reduction of NOx while the engine is operated under lean
conditions. The NOx trap functions in a cyclic manner. When the NOx
trap reaches the effective storage limit, the engine is operated
under normal or rich conditions to purge the NOx trap. After the
NOx trap has been purged, the engine can return to lean
operation.
[0010] However, in addition to problems associated with thermal
stability and sulfur tolerance, lean NOx traps (LNT) have the
following two known problems: (1) a problem referred to as "NOx
breakthrough", the breakthrough of NOx during the NOx trap
transition from the lean to the rich cycle; and (2) a reduction in
fuel economy that results from frequent purges during the rich
cycle. Test results depicted in FIG. 2 shows this NOx breakthrough
for LNTs with different oxygen storage capacity (OSC). This total
NOx breakthrough has been found to be greater than 73% of the total
NOx emitted during the operation of a lean NOx trap.
[0011] FIG. 2 also shows the effects of oxygen storage capacity on
NOx breakthrough of a lean NOx trap during the lean-to-rich
transition. LNT L, which has the highest OSC, results in the
largest amount of NOx breakthrough, while the lower the OSC (from
LNT M down to LNT N), the lower the amount of NOx breakthrough. It
is believed that NOx breakthrough during the lean-rich transition
occurs due to the exothermic heat generated from the oxidation of
reductants, CO, HC and H.sub.2, by the oxygen released from the
oxygen storage material (see FIG. 2)--the temperature rise can be
as high as 80-100.degree. C.
[0012] With regard to the fuel economy penalty, this is believed to
be the result of high oxygen storage capacity of the lean NOx trap,
low NOx trapping capacity, and/or high exhaust flow rate. The OSC
requires additional reductants (i.e., fuel) to reduce the oxygen
storage materials during each lean-to-rich transition, while the
low NOx trapping capacity requires that the frequency of purges be
increased.
[0013] The present invention avoids the cost and complexity of the
NOx trap and the reduced fuel economy from frequent NOx trap
purging by systematically reducing the amount of NOx during engine
operation, even under lean conditions.
[0014] To solve the above problems, the present invention provides
a new catalyst system comprising two catalysts that can treat all
exhaust emissions, CO, HC and NOx under both stoichiometric and
lean conditions. In particular, the forward catalyst uses a newly
developed Perovskite-based formulation which achieves the requisite
close proximity between precious metal and NOx binding
elements.
[0015] The closest known prior art includes modified three-way
catalysts. For example, U.S. Pat. No. 4,024,706, incorporated
herein by reference, teaches a method of enlarging the air/fuel
ratio over which a catalyst operates by including an oxygen storage
material. The method involves controlling the air/fuel ratio of the
fuel mixture being burned by the engine such that the ratio is
transferred into equal amounts going to the rich and lean side of a
stoichiometric condition as previously described. The use of an
oxygen storage material, however, is believed to result in NOx
breakthrough, which increases NOx emissions rather than reducing
them.
[0016] U.S. Pat. No. 5,977,017 teaches a Perovskite-type catalyst
that consists mainly of a metal oxide composition. The metal oxide
composition is represented by the general formula:
A.sub.a-xB.sub.xMO.sub.b, where
[0017] A is a mixture of elements originally in the form of a
single phase mixed lanthanides collected from bastnasite;
[0018] B is a divalent or monovalent cation;
[0019] M is at least one element selected from the group consisting
of elements of an atomic number from 22-30, 40-51, and 73-80;
[0020] a is 1 or 2;
[0021] b is 3 when a is 1 or b is 4 when a is 2; and
[0022] x is between 0 and 0.7.
[0023] This general Perovskite structure, however, is not designed
to maximize NOx storing and releasing functions--by providing the
requisite close proximity between the precious metal and the
NOx-binding element. In contrast, the newly developed Perovskite
structure of this invention is specifically designed to maximize
NOx storage and release.
[0024] U.S. Pat. No. 4,321,250 also teaches a Perovskite-type
catalyst having a ABO.sub.3 crystal structure with about 1 to 20
percent of the B cation sites occupied by Rh ions and the remainder
of the B cation sites occupied by ions consisting essentially of
cobalt and the A cation sites occupied by lanthanide ions of atomic
number 57 to 71 and ions of at least one metal of groups IA, IIA or
IVA of the period table having an ionic radii of about 0.9 A to
1.65 A and proportioned so that no more than 50 percent of the
cobalt ions are tetravalent and the remaining cobalt ions are
trivalent. This composition generally represents Perovskite
catalysts that were useful to produce hydrogen in steam
reformers.
[0025] The use of such types of Perovskite catalysts as an
automotive catalyst or their use in combination with other
catalysts to produce a NOx tolerant catalyst was not known prior to
this invention.
SUMMARY OF THE INVENTION
[0026] The present invention is directed to a catalyst system for
use with an internal combustion engine. In broad terms, the
catalyst system of this invention is designed specifically to
maximize reduction of NOx emissions during lean exhaust conditions.
The catalyst system can be a single catalyst or a combination of
two or more catalysts. Whether one catalyst is used or more than
one, the catalyst system is designed to maximize reduction of NOx
emissions under lean conditions, and maximize the reduction of HC,
NOx and CO emissions under stoichiometric conditions.
[0027] More specifically, this invention relates to a new
Perovskite formulation for a catalyst to be used in a catalyst
system that maximizes the reduction of HC, CO, and NOx under both
stoichiometric and lean operating conditions. This new
Perovskite-based formulation is to be used in the forward catalyst
of a catalyst system, which also uses a downstream catalyst,
wherein the forward catalyst is used primarily to reduce emissions
under lean operating conditions, and wherein the downstream
catalyst is designed primarily to reduce emissions under
stoichiometric conditions. It is believed that the newly designed
Perovskite-based forward catalyst is optimized for reducing
emissions under lean operating conditions by achieving close
proximity between the precious metal and NOx-binding elements, such
as barium, magnesium and potassium.
[0028] We have found that a catalyst system of this construction
and composition is capable of oxidizing hydrocarbons and carbon
monoxide while also reducing NOx during systematic operation under
lean conditions. Accordingly, this invention provides combined
treatment of emissions of engines operated under both
stoichiometric conditions and lean burn conditions, and provides
excellent thermal stability and the resulting metal dispersion
eliminates NOx breakthrough during the lean to rich transition.
This and other aspects of the invention will be described in detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a graph of engine speed versus brake mean
effective pressure (BMEP) at different air/fuel ratios for typical
internal combustion engines and the proposed new stratified-charged
engine;
[0030] FIG. 2 is a graph of NOx released during the transition from
lean-rich operation in milliseconds for three lean NOx traps with
different oxygen storage capacities (OSC) in a flow reactor at
350.degree. C.;
[0031] FIG. 3 is a schematic view of a catalyst system that
incorporates the present invention, showing two catalysts and an
EGO sensor positioned therebetween to maximize the treatment of
emissions both under stoichiometric operation and under
stratified-charged lean conditions;
[0032] FIG. 4 is a schematic diagram of the Perovskite crystal
structure;
[0033] FIG. 5a shows the NOx trapping efficiency of the catalyst as
prepared in Example 1; and
[0034] FIG. 5b shows the conversion efficiency of the catalyst as
prepared in Example 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0035] Demands for improved fuel economy and lower CO.sub.2
emissions have encouraged engine manufacturers to increase the
air/fuel ratio above 14.7--above stoichiometric conditions.
[0036] In one newer engine design, the engine is run under
stoichiometric conditions most of the time, except under low load
(brake mean effective pressure (BMEP)<1.2 bar), low engine speed
(RPM<1750) conditions, when the engine is run under
stratified-charged lean conditions, an air/fuel ratio of
approximately 30. The operation diagram of a stratified charged
(SC) engine is schematically shown as FIG. 1. FIG. 1 depicts the
operation regions of typical internal combustion engines,
homogeneously charged engines, with an air/fuel ratio=28, an engine
operating under stoichiometric conditions, an air/fuel ratio of
14.6 and an engine operating at full load .lamda.<1, wherein
.lamda. is the air/fuel excess ratio. It is predicted that this
engine operation would increase fuel economy by approximately 5%.
For such engines, designed to operate at least partially under lean
conditions, the present invention provides a catalyst system
capable of reducing CO, HC, and NOx--in line with current and
future emission standards.
[0037] As seen in FIG. 3, under this invention, one embodiment of
the catalyst system 10 includes two closely-coupled catalysts--a
forward catalyst 12 to maximize the reduction of engine 16
emissions under lean conditions and a downstream catalyst 14 to
maximize the reduction of emissions under stoichiometric
conditions. In particular, the forward catalyst 12 of this
invention is comprised of a newly developed Perovskite composition
designed to increase proximity between the precious metal and the
NOx binding metals and to eliminate the oxygen storage
capacity.
[0038] In general, Perovskites have the generic formula ABO.sub.3.
Perovskites can be classified into three categories according to
the valence of the A and B elements: A.sub.1B.sub.5O.sub.3,
A.sub.2B.sub.4O.sub.3, and A.sub.3B.sub.3O.sub.3. A schematic
crystal structure of a Perovskite, CaTiO.sub.3, is shown in FIG. 4.
As can be seen in FIG. 4, the B element is at the octahedral
interstitial site at the center of the unit cell. As a result, B
generally has a smaller ionic radius than A. In fact, an empirical
tolerance factor defines the relationship between the ionic radius
of A and the ionic radius of B: T=(r.sub.A+r.sub.O)/1.4144
(r.sub.B+r.sub.O), T must satisfy 0.75 less than T less than
1.00.
[0039] The active sites for a Perovskite are normally the B sites.
For a reaction NO+CO to occur, the site should have balanced
oxidation and reduction activity. Too strong an oxidation activity
will drive CO immediately to CO.sub.2 without involving NO, and too
strong a reduction activity will drive NO to NOC or N.sub.2O
without converting CO to CO.sub.2. We have found that cobalt, Co,
has this balanced oxidation/reduction property, and thus be placed
on the B site. We have also found that lanthanum, La, at the A site
provides adequate stability for the Perovskite. As a result, in
this invention, the preferred parent Perovskite structure is
LaCoO.sub.3.
[0040] For NOx storing and releasing to occur, high capacity, fast
storing and releasing kinetics is further required. This is
accomplished by substituting barium, Ba, to the lanthanum, La, (A)
sites to form La.sub.1-xBa.sub.xCoO.sub.3. Since lanthanum is
trivalent while barium is divalent, the charge balance of the
crystal structure results in the formation of either a tetravalent
cobalt or positive holes (oxygen vacancies):
La.sup.3t.sub.1-xBa.sup.2t.sub.xCo.sup.3t.sub.1-xCo.sup.4t.sub.xO.sub.3
La.sup.3t.sub.1-xBa.sup.2t.sub.xCo.sup.3tO.sub.3-x/2VO.sub.x/2
[0041] By substituting barium with lanthanum, the oxygen vacancies
created provide space for bulk nitrate or nitrite formation, to
achieve NOx storage, and also accelerate the diffusion of nitrogen
atoms inside the Perovskite structure.
[0042] For this invention, part of the lanthanum at the A site can
also be substituted with magnesium, Mg, and potassium, K, to
provide balanced trapping function at both low and high
temperatures. Additionally, the lanthanum at the A site can be
substituted with Sr.
[0043] In a preferred embodiment, the parent Perovskite structure
LaCoO.sub.3 can be modified, wherein cobalt is substituted at the B
site with precious metals such as platinum and rhodium, or a
transition metal such as iron, copper, and manganese--to increase
the activity and selectivity of the Perovskite structure. The
substituted metal, i.e., platinum, will thus have close proximity
with barium at the molecular level which improves the thermal
stability and NOx reducing capabilities of the forward catalyst 12,
as shown in FIG. 3.
[0044] This newly developed Perovskite catalyst composition can
generally be prepared according to a sol-gel method. The Perovskite
composition can be coated onto a block of honeycomb cordierite
substrate (600 csi). After the Perovskite is coated, extra platinum
and rhodium can then be impregnated onto the coated substrate in a
ratio of 7:1. The platinum and rhodium are loaded at approximately
20-100 g/ft.sup.3.
[0045] In one preferred embodiment, the Perovskite used as the
forward catalyst has a formula of:
La.sub.0.5Ba.sub.0.5Co.sub.0.9Pt.sub.0.1O.sub.3. This preferred
Perovskite composition can be prepared using a solution of citric
acid and ethylene glycol. More specifically, 0.667 gm of citric
acid and 4 cm.sup.3 of ethylene glycol per 1 gm of the final oxide
mixture was added to a boiling solution of La, Ba, Co nitrates and
tetra-amine platinum nitrate in the desired ratios. The resulting
mixture is evaporated with vigorous stirring until formation of a
gel, then further evaporated on a hot plate to remove the residual
liquid. The resulting powder was ground and heated up to
300.degree. C. for six hours, to remove the organic matter and then
ground again and calcined at 900.degree. C. for 30 hours in
air.
[0046] Another preferred Perovskite composition has the formula:
La.sub.0.5Ba.sub.0.5Co.sub.0.9Rh.sub.0.1O.sub.3
[0047] Yet another preferred Perovskite composition has the
formula:
La.sub.0.5Ba.sub.0.5Co.sub.0.6Fe.sub.0.3Pt.sub.0.1O.sub.3
[0048] It should be noted that the above Perovskite structures can
also be prepared using the co-precipitation method.
Co-precipitation techniques are well known to those skilled in the
art. According to such techniques, the soluble salts can be
dissolved in a solvent, for example, nitrates of the metals are
dissolved in water. Co-precipitation is then obtained by making the
solution basic, e.g., a pH of 9 by adding a base like ammonium
hydroxide. Other soluble metal compounds such as, for example,
sulfates and chlorides, may be used as may mixtures or various
soluble compounds, e.g., nitrates with chlorides. The precipitate
would then be heated to decompose it to the mixed metal oxide. This
heating and calcination can be carried out at temperatures of up to
900.degree. C. It should be noted that the way in which the oxide
is obtained for use in forming the catalyst is not critical to the
invention. Still other ways and other soluble salts would be
apparent to those skilled in the art in view of the present
disclosure.
[0049] As is known in the art, for useful application of the
catalyst in an exhaust gas system, the catalyst is deposited or
washcoated on a substrate (mechanical carrier) made of a high
temperature stable, electrically insulating material such as
cordierite, mullite, etc. A mechanical carrier is preferably
comprised of a monolithic magnesium aluminum silicate structure,
i.e., cordierite, although the configuration is not critical to the
catalyst of this invention.
[0050] It is preferred that the surface area of the monolithic
structure provide 50-1000 meters square per liter structure, as
measured by nitrogen adsorption. Cell density should be maximized
consistent with pressure drop limitations and is preferable in the
range of 200-800 cells per square inch of cross-sectional area of
the structure. The substrate may be in any suitable configuration,
often being employed as a monolithic honeycomb structure, spun
fibers, corrugated configurations useful in this invention and
suitable in an exhaust gas system will be apparent to those skilled
in the art in view of the present disclosure.
[0051] Techniques for providing an oxide washcoat on a substrate
are well known to those skilled in the art. Generally, a slurry of
the mixed metal oxide particles and optionally stabilizer particles
are coated on a substrate, e.g., added by dipping or spraying,
after which the excess is generally blown off. After the slurry of
mixed metal oxide particles are coated on the substrate, the
substrate is heated to dry and calcine the coating, generally at a
temperature of about 700.degree. C. for about 2-3 hours. Calcining
serves to develop the integrity of the ceramic structure of the
washcoated oxide coating. The total amount of the oxide washcoat
carried on the substrate is about 10-40% (wt), based on the weight
of the substrate coated. Several coatings of the substrate and the
washcoat may be necessary to develop the desired coating
thickness/weight on the substrate.
[0052] For the downstream catalyst 14 of FIG. 3, the basic
formulation is a catalyst that can include excess Ba compounds to
stabilize the alumina carrier. Precious metals may also be provided
on the calcined oxide coating by any technique including the
well-known wet impregnation technique from soluble precious metal
precursor compounds. Water soluble compounds are preferred,
including, but not limited to nitrate salts and materials for
platinum like chloroplatinic acid. As is known in the art, after
impregnating the washcoat with the precursor solution, it is dried
and heated to decompose the precursor to its precious metal or
precious metal oxide. As is known in the art, the precursor may
initially decompose to the metal but be oxidized to its oxide in
the presence of oxygen. While some examples of precious metal
precursors have been mentioned above, they are not meant to be
limiting. Still other precursor compounds would be apparent to
those skilled in the art in view of the present disclosure.
[0053] In addition to this incorporation from a liquid phase, the
precious metal, such as platinum, may be provided by sublimation of
platinum chloride or other volatile platinum salts, by a solid
state exchange in the 300-500.degree. C. temperature range using
labile platinum compounds. There is no criticality to the type of
precursor compounds that may be used to provide the precious metal
according to this invention.
[0054] FIG. 3 depicts one embodiment of the catalyst system 10 of
the present invention. As shown, the catalyst system 10 includes
two catalysts 12, 14 in a close-coupled location. The forward
catalyst 12 is optimized to function when the engine 16 is operated
under lean conditions. The forward catalyst 12 will store excess
NOx during lean operation and then release and convert the NOx when
the engine 16 switches to stoichiometric or rich conditions. The
downstream catalyst 14 is optimized to effectively convert HC, CO,
and NOx under stoichiometric operations.
[0055] The forward catalyst 12 includes the newly developed
Perovskite composite. By using this Perovskite composition, NOx
emissions during lean conditions are reduced. This reduction is
believed to be the result of the newly developed Perovskite
composition which places the precious metal and NOx binding metals
in close proximity.
[0056] The downstream catalyst 14 comprises a catalyst mixture
PM-Rh, where PM (precious metal) is a catalyst material selected
from the group consisting of platinum, palladium and combinations
thereof and the PM is then mixed with Rh (rhodium) to form the
downstream catalyst 14 catalyst mixture. This downstream catalyst
14 also preferably comprises an alumina substrate, on which the
PM-Rh catalyst mixture is coated.
[0057] In a preferred embodiment, the downstream catalyst 14
contains platinum and rhodium, in a ratio of Pt/Rh 9:1, and more
preferably 7:1. The total loading of the precious metal (PM) in the
downstream catalyst 14 is, however, about 20-60 g/ft.sup.3, and
more preferably 40-60 g/ft.sup.3, which results in a cost savings
compared to these catalysts which have a precious metal loading of
approximately 100-120 g/ft.sup.3. In this preferred embodiment,
both Pt and Rh are anchored on 2-20% (wt) high surface area Ce/Zr
with high O.sub.2 kinetics (e.g., Ce/Zr=50:50 molar ratio). The
alumina washcoat is preferably also stabilized by 2-20% BaO.
[0058] The foregoing catalyst system 10 eliminates the oxygen
storage function of the forward catalyst 12, which is normally
present in lean NOx trap formulations, so that NOx breakthrough is
minimized. The forward catalyst 12 can be purged and NOx converted
when an engine control module (ECM) senses that the engine is under
high or low load, corresponding to acceleration or deceleration,
respectively.
[0059] Optionally, an exhaust gas oxygen sensor 18 is positioned
upstream of the downstream catalyst 14 between catalyst 12 and
catalyst 14, as shown in FIG. 3. Under this arrangement, there is
no fuel economy penalty from the offset of the oxygen storage
capacity of the forward catalyst because we have eliminated the
oxygen storage capacity in the forward catalyst.
[0060] This catalyst system is expected to be used in automotive
vehicles for emission treatment in the exhaust gas system where it
functions to oxidize hydrocarbons, carbon monoxide, and
simultaneously reduce nitrogen oxides to desired emission
levels.
[0061] One alternative embodiment of this invention uses just the
Perovskite forward catalyst 12 to reduce emissions from the exhaust
streams of such two-cylinder machines/devices as boats, jet skis,
lawn mowers or cutting devices to provide a low cost catalyst.
Under this embodiment, the Perovskite catalyst would be coated
directly on the exhaust emitting component of the device, i.e., a
muffler, or alternatively coated on an inexpensive substrate, such
as ceramic. To ensure that the catalyst remains low cost, the
cobalt B cation sites on the Perovskite crystal structure would be
substituted with a metal selected from the group consisting of
iron, copper and manganese--not a precious metal. It should be
noted that for such Perovskite applications, low sulfur content
fuel is preferred to avoid sulfur poisoning of the Perovskite
catalyst.
EXAMPLE 1
[0062] La(NO.sub.3).sub.3.6H.sub.2O (108.25 g), Ba(NO.sub.3).sub.2
(65.34 g), Co(NO.sub.3).sub.2.6H.sub.2O (130.97 g), and
Pt(NH.sub.3).sub.4 (NO.sub.3).sub.2 (19.35 g) are each added to 500
ml deionized water, heated to 100.degree. C., and then mixed
together to achieve a solution with the final desired ratios. This
stirred solution is heated and allowed to boil before adding a
solution containing 0.667 g of citric acid and 4 cm.sup.3 of
ethylene glycol per 1 g of the final oxide mixture. The resulting
mixture is evaporated with vigorous stirring until formation of a
gel, and then further evaporated on a hot plate at 140.degree. C.
to remove the residual liquid. The resulting powder is ground and
heated to 300.degree. C. for 6 hours and allowed to cool to room
temperature. The powder is ground again and then calcined in air at
900.degree. C. for 30 hours. The final powder composition is
La.sub.0.5Ba.sub.0.5Co.sub.0.9Pt.sub.0.1O.sub.3.
EXAMPLE 2
[0063] The sample is prepared by the same method as described in
Example 1 with the exception of adding 16.25 g of
Rh(NO.sub.3).sub.3.2H.sub.2O to 500 ml deionized water instead of
Pt(NH.sub.3).sub.4 (NO.sub.3).sub.2. The resulting powder is
La.sub.0.5Ba.sub.0.5Co.sub.0.9Rh.sub.0.1O.sub.3.
EXAMPLE 3
[0064] La(NO.sub.3).sub.3.6H.sub.2O (108.25 g), Ba(NO.sub.3).sub.2
(65.34 g), Co(NO.sub.3).sub.2.6H.sub.2O (87.31 g),
Fe(NO.sub.3).sub.3.9H.sub.2O (60.60 g), and Pt(NH.sub.3).sub.4
(NO.sub.3).sub.2 (19.35 g) are each added to 500 ml deionized
water, heated to 100.degree. C., and then mixed together to achieve
a solution with the final desired ratios. This stirred solution is
heated and allowed to boil before adding a solution containing
0.667 g of citric acid and 4 cm.sup.3 of ethylene glycol per 1 gm
of the final oxide mixture. The resulting mixture is evaporated
with vigorous stirring until formation of a gel, and then further
evaporated on a hot plate at 140.degree. C. to remove the residual
liquid. The resulting powder is ground and heated to 300.degree. C.
for 6 hours and allowed to cool to room temperature. The powder is
ground again and then calcined in air at 900.degree. C. for 30
hours. The final powder composition is
La.sub.0.5Ba.sub.0.5Co.sub.0.6Fe.sub.0.3Pt.sub.0.1O.sub.3.
[0065] The foregoing catalyst systems constructions and
compositions have been found useful in reducing harmful engine
emissions. Variations and modifications of the present invention
may be made without departing from the spirit and scope of the
invention or the following claims.
* * * * *